A conventional liquid crystal display apparatus is used for a display section of an electronic information device, such as a cell phone, a personal digital assistance (PDA), a laptop personal computer and the like. This liquid crystal display apparatus includes a liquid crystal display panel in which a liquid crystal layer is interposed between a pair of substrates. A display voltage is applied to electrodes provided on both substrates so as to change an orientation state of liquid crystal molecules in the liquid crystal layer, thereby scattering/passing the light. As a result, a character/figure is displayed.
Unlike cathode ray tube (CRT) and electroluminescence (EL) display panels, the liquid crystal display panel does not emit light by itself. Therefore, a transmissive liquid crystal display apparatus is commonly used in which a light transmissive material is used for a pixel electrode. The transmissive liquid crystal display apparatus controls a transmissive amount of light from a backlight, which is provided on the back side of a liquid crystal display panel, using the liquid crystal panel so as to produce an image display.
However, in a conventional transmissive liquid crystal display apparatus, the backlight usually occupies 50% or more of the total power consumption of the liquid crystal display apparatus. Thus, there is a problem that providing a backlight increases power consumption. In addition, in the transmissive liquid crystal display apparatus, when surrounding light is extremely bright, display light looks darker than the surrounding light, thus causing a problem of making it difficult to recognize the display.
As such, in a portable electronic information device which is always carried, for example, outside and used in many occasions, a reflective liquid crystal display apparatus is used in which a reflective light is provided instead of the backlight or a light reflective material is used for a pixel electrode. The reflective liquid crystal display apparatus reflects the surrounding light from the front side of a liquid crystal display panel using the light reflective material so as to produce a display.
However, in a conventional reflective liquid crystal display apparatus, since the reflective light of the surrounding light is used, a problem is caused of extremely reducing the visibility when the amount of the surrounding light is small. In addition, in the reflective liquid crystal display apparatus, the display is produced utilizing the surrounding light for the purpose of reducing the power consumption. Therefore, even in an environment in which power can be sufficiently supplied, if the surrounding light is darker than a predetermined limit value, a problem is caused in that the display cannot be recognized.
As such, recently, a semi-transmissive reflective liquid crystal display apparatus having a transmissive section and a reflective section provided in one pixel has been used, which can produce both a transmissive display and a reflective display.
A conventional semi-transmissive reflective liquid crystal display apparatus includes a liquid crystal display panel having a liquid crystal enclosed between an element-side substrate and an opposing-side substrate, wherein a plurality of pixel electrodes arranged in a matrix and switching elements, such as thin film transistors (TFT) for selectively driving the pixel electrodes, are provided on the element-side substrate, and a plurality of opposing electrodes opposing the plurality of pixel electrodes are provided on the opposing-side substrate. The pixel electrode includes a reflective electrode for reflecting the surrounding light and a transmissive electrode for passing the light from a backlight. The surrounding light is reflected by the reflective electrode and then irradiated onto a liquid crystal layer, and also, the light emitted from the backlight is transmitted through the transmissive electrode and then irradiated onto the liquid crystal layer.
However, in the conventional semi-transmissive reflective liquid crystal display apparatus, the trans missive electrode and the reflective electrode are made of different metal materials, and thus the electrical properties for the respective electrodes are different from each other. As such, the optimum value of a direct current offset voltage (optimum opposing voltage) which is applied to offset a bias electric field generated inside the liquid crystal is different in the transmissive electrode and the reflective electrode.
Therefore, when the optimum direct current offset voltage is applied to one of the transmissive electrode and the reflective electrode, a problem is caused of deteriorating the display quality due to a display flicker, resulting from a difference with the optimum direct current offset voltage for the other electrode. In addition, applying a direct current voltage component for a long time causes a problem of deteriorating the reliability of the liquid crystal.
These problems are considered to result from the fact that the difference of the work functions in the transmissive electrode and the reflective electrode is large (0.4 eV) since the value of the work function of a transparent metal oxide, such as Indium Tin Oxide (ITO), used for a material of the transmissive electrode is about 4.7 eV to 5.2 eV and the value of the work function of a metal material, such as aluminum (Al), used for a material of the reflective electrode is about 4.2 eV to 4.3 eV.
For example, Reference 1 discloses a semi-transmissive reflective liquid crystal display apparatus, in which a transparent electrode material layer having the same work function as that of an opposing electrode is formed on a reflective electrode, made of aluminum, via an insulation layer, and the transparent electrode material layer is also formed as a transparent electrode in a transmissive region.
In addition, Reference 2 and Reference 3 disclose a reflective liquid crystal display apparatus in which a transparent electrode material layer having the same work function as that of an opposing electrode is formed on a reflective electrode.
According to these conventional techniques, by forming the transparent electrode material layer having the same work function as that of the opposing electrode on the reflective electrode, it is possible to eliminate the difference of the optimum values of the direct current offset voltages resulting from the difference with the work function of the opposing electrode and also possible to prevent the deterioration of the display quality and the deterioration of the reliability of the liquid crystal.
[Reference 1] Japanese Laid-Open Publication No. 2003-255375 (page 3 to page 5)
[Reference 2] Japanese Laid-Open Publication No. 10-206845
[Reference 3] Japanese Laid-Open Publication No. 2002-365664